Various types of measuring systems are known to determine the geomagnetic field, for use in navigation. When applied to automotive or land vehicles, the stray fields, for example due to the chassis of a vehicle, or magnetic fields due to connected equipment, have the tendency to distort the geomagnetic field. It has previously been proposed--see German Pat. No. 27 54 888--to measure magnetic fields due to the vehicle or equipment installed therein, and to correct the sensor output by a respective correction value, or, alternatively, to determine the direction--vectorially--of the stray fields which affect the sensor. The stray fields are vectorially, directionally dependent. A typical magnetic field sensor is an element which responds in two orthogonal axes. The level of the magnetic field within the range of possible installations of the sensors may change substantially even upon moving of the sensor only by small distances, for example a few centimeters. In an automotive vehicle, a sensor is usually located either behind the dashboard, beneath the roof of the vehicle, or in the range of a rear window shelf. If an optimum location is determined for any particular type of vehicle, it is not a necessary consequence that all vehicles of this type will have the same stray fields generated at a specific location since, even in vehicles of the same type, different directions of drawing or rolling the sheet metal components thereof may differentially affect the sensor. Thus, the stray field at similiar locations even in vehicles of the same type may be different. Upon installation of a sensor at a location where the stray field is high, the magnetic field sensor may be subjected to fields which cause the sensor to saturate, resulting in erroneous output indications and output signals therefrom.
It is an object to provide a method to determine the optimum location for a magnetic field sensor in an automotive vehicle, and specifically at a location where the stray or disturbance field is so small that the sensor will not be subjected to saturation phenomena, and erroneous results will be avoided.
Briefly, a field sensor is preliminarily installed in a vehicle at a first location, and the vehicle is then rotated by 360.degree.. During rotation of the vehicle, the output from the field sensor is measured in an evaluation circuit, and minimum and maximum values therefrom are determined; these minimum and maximum values are representative of the entire field to which the field vector is subjected. The level of the disturbance field is then determined by adding, in the respective vectorial directions, half of the respective maximum and minimum values. The foregoing steps are then repeated at a different location of the sensor. The respective results are stored or noted and, when a location is determined which results in minimum level of disturbance, the sensor is then securely affixed to the vehicle at that point.
The method has the advantage that a magnetic field sensor need not be permanently installed in the vehicle at a location where the disturbance field may be substantial; rather, in advance of the permanently fixed installation, the sensor is preliminarily installed and the specific sensor itself determines the field. External or special measuring elements are not required. The disturbance field should, preferably, be less than the geomagnetic field on the basis of which navigation is to be carried out; the method has the additional advantage that it is readily possible to select, from all possible locations, that one in which the ratio of geomagnetic field to disturbance field provides optimum results.
In accordance with a feature of the invention, a repetitive sequence, by trial and error, of locations of the disturbance field is carried out until at least at one location the field strength of the disturbance field is at the most equal, and preferably less than half the saturation field of the respective magnetic field sensor with respect to the X and Y axes of a vectorial diagram. When using an electronic compass with a computer, it is particularly desirable to provide a suitable computer program which, during rotation of the vehicle by 360.degree., that is, about one revolution, continuously determines the entire field vector and, hence, the maximum and minimum value of the entire field vector. The maximum and minimum values are then stored in a buffer store or the like, for example, which is addressed to calculate therefrom the level of the disturbance field in accordance with a simple and well known mathematical formula. The level can be displayed on a display. This permits, in the simplest manner, determining the level of the disturbance field, and the maximum and minimum value of the overall field vector with respect to the X-axis and the Y-axis, respectively, of the vectorial diagram with the associated values respecting the X and Y-axes.
Magnetic fields are generated not only by the fixed or permanent metal components of the vehicle but, additionally, may be transient or intermittent due to operation of electrical equipment within the vehicle. For example, electric motors, electromagnetic relays, or even electrical rear window defroster heaters and the like can generate magnetic fields which affect the magnetic field sensor. Particularly rear window defrosters, located within the rear window of a vehicle, may affect a magnetic field sensor located beneath the rear window shelf. In order to exclude disturbances from such electrical equipment, a location-determination cycle is carried out with the electrical equipment energized, and the disturbance field determined as described above. The sequence is then repeated and, if due to connection of electrical equipment or appliances, the disturbance field changes to an impermissible extent, the previously selected installation location can be changed and other, better locations determined by repeating the preliminary installation, rotation, and disturbance field determination steps.